Video Compression Using FFmpeg (HEVC / H.265 & H.264)

This note explains how to compress videos using FFmpeg with HEVC (H.265) and H.264 (AVC), focusing on broad compatibility across devices — PCs, TVs, ARM devices, mobile, and browsers.

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1. Recommended Format & Codec

Parameter	Recommendation
Container	MP4 (.mp4) — universal support
Video Codec	HEVC / H.265 (hvc1) or H.264 (avc1)
Audio Codec	AAC LC
Audio Bitrate	128–192 kbps
Resolution	Match target device (1080p, 720p)
Frame Rate	Keep source unless downscaling
Chroma / Bit Depth	4:2:0, 8-bit (broad compatibility)

HEVC gives smaller file sizes at same quality; H.264 maximizes legacy device support.

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2. Key FFmpeg Concepts

🔹 Video Scaling

-vf scale=WIDTH:-2

• Adjusts width, keeps aspect ratio (height auto, even number)

🔹 Rate Control / Quality

Option	Meaning
-rc:v vbr	Variable bitrate, adjusts bitrate per scene complexity
-cq:v N	Constant quality target (lower = higher quality, bigger file)
-crf N	Constant Rate Factor (libx265 / libx264), lower = higher quality

HEVC CQ examples (1080p):

• 18–22 → very high quality
• 23–27 → balanced quality & size
• 28–30 → smaller file, some compression artifacts

Max Bitrate Control

-b:v 3000k -maxrate 3500k -bufsize 7000k

• -b:v → target bitrate
• -maxrate → max bitrate peak
• -bufsize → VBV Buffer Size (Rate Control Window)
  • Common rule: bufsize = 2 × maxrate → avoids quality drops

🔹 Fast Start

-movflags +faststart

• Enables streaming / instant playback.

🔹 Codec Tag

• HEVC → -tag:v hvc1 for iOS / modern player support
• H.264 → MP4 uses avc1 by default

🔹 Audio

• AAC LC, 128–192 kbps → universal support

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3. FFmpeg Quick Reference

NVENC HEVC (modern devices, smaller size)

ffmpeg -y -hwaccel cuda -i input.mp4 \
    -map 0:v -vf scale=1920:-2 \
    -c:v hevc_nvenc -preset p7 -rc:v vbr -cq:v 25 -tag:v hvc1 \
    -map 0:a -c:a aac -b:a 160k \
    -movflags +faststart \
    output_hevc.mp4

Explanation:

• -hwaccel cuda → NVIDIA GPU acceleration
• -map 0:v → Select all video streams
• -vf scale=1920:-2 → Scale width to 1920px, auto height to maintain aspect ratio
• -c:v hevc_nvenc → Use NVENC hardware HEVC encoder
• -preset p7 → slowest NVENC preset → best quality / smallest file
• -rc:v vbr → Variable bitrate, adjusts to scene complexity
• -cq:v 25 → Constant quality target (lower = higher quality, bigger file)
• -tag:v hvc1 → HEVC tag for MP4, ensures broad device compatibility
• -map 0:a -c:a aac -b:a 160k → Encode all audio streams to AAC 160 kbps
• -movflags +faststart → Allows streaming / instant playback

HEVC Remux (already HEVC, fix compatibility)

ffmpeg -i input.mp4 -c copy -tag:v hvc1 -movflags +faststart output_fixed.mp4

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NVENC H.264 (maximum legacy compatibility)

ffmpeg -y -hwaccel cuda -i input.mp4 \
    -map 0:v -vf scale=1920:-2 \
    -c:v h264_nvenc -preset p7 -rc:v vbr -cq:v 23 -profile:v high -level 4.0 \
    -map 0:a -c:a aac -b:a 160k \
    -movflags +faststart \
    output_h264.mp4

Explanation:

• -c:v h264_nvenc → NVENC hardware H.264 encoder
• -profile:v high -level 4.0 → Ensures compatibility with most devices (TVs, browsers, PCs)
• Other options same as HEVC example, optimized for H.264

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CPU H.265 / x265 (libx265)

ffmpeg -i input.mp4 \
    -c:v libx265 -preset slow -crf 24 \
    -c:a aac -b:a 160k \
    -movflags +faststart \
    output_x265.mp4

Explanation:

• -crf 24 → constant quality, smaller CRF = better quality, larger file
• Preset slow → better compression
• CPU encoding is slower than NVENC but may yield slightly smaller files at same quality

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CPU H.264 / x264 (libx264)

ffmpeg -i input.mp4 \
    -c:v libx264 -preset slow -crf 23 -profile:v high -level 4.0 \
    -c:a aac -b:a 160k \
    -movflags +faststart \
    output_x264.mp4

Explanation:

• CRF 23 → good balance of quality and file size
• High profile + level 4.0 → maximum compatibility

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4. Tips & Recommendations

1. HEVC vs H.264
   • HEVC → modern devices, smaller file
   • H.264 → maximum compatibility
2. CQ / CRF
   • Lower = higher quality, larger file
   • Higher = smaller file, may show artifacts
3. Audio → AAC LC 128–192 kbps
4. Streaming & Web → always use -movflags +faststart
5. Scaling → keep width/height divisible by 2 (-2 auto-fixes height)
6. Check FFmpeg options

   ffmpeg -h encoder=hevc_nvenc
   ffmpeg -h encoder=h264_nvenc
   ffmpeg -h encoder=libx265
   ffmpeg -h encoder=libx264